The present invention relates to a microwave apparatus and method for heating materials, objects, liquids or the like under pressure. In one aspect, the invention can be used for the sterilization of medical wastes by means of microwave energy used to generate sufficient heat and pressure to sterilize. While the invention will be described in relation to the preferred embodiment tailored for medical wastes, it will be apparent that it is suitable for a wide range of applications.
Medical waste is infectious refuse that can transmit a wide variety of diseases. Generators of medical waste include hospitals, doctors' offices, clinics, dental offices, laboratories, research facilities, nursing homes, and funeral parlors. As medical costs rise, an ever-increasing number of long term illnesses are being treated at home, the result of which is medical waste mixed with ordinary household trash. Additionally, the quantity of medical waste has dramatically increased in recent years due to the expanded use of disposable, rather than reusable, products.
In many jurisdictions, the definition of medical waste has been broadened to include an ever-widening variety of materials. The Center for Disease Control has issued recommended procedures whereby any material that comes into contact with any patient's body fluids be treated as if it were infectious. Examples of such waste include bandages, gloves, tubing, syringes, laboratory cultures, and pathological wastes. Furthermore, in response to an increase in illegal medical waste disposal, most states have enacted statutes and regulations on the handling of medical waste from large- and small-scale generators. In addition, the federal Medical Waste Tracking Act created a lifetime tracking system for infectious waste. Before sterilized waste can be disposed of as noninfectious, it must be rendered unrecognizable compared to its original form.
Typically, waste which is generated is isolated in special sealed containers until it can be treated. Except for using chemical treatment, all infectious bacteria, viruses and organisms are normally destroyed by some form of heat. The most widely used forms of heat treatment are autoclaving, i.e., sterilization with steam, and burning at specially equipped incineration sites. Both retrofitting existing hospital incinerators and building new environmentally acceptable incinerators are extremely costly alternatives, while conventional autoclaving is often too slow and costly. Conversely, on-site treatment greatly reduces disposal costs because the material need not be treated as hazardous. A rapid, on-site, low cost treatment for sterilizing or otherwise treating medical waste is, therefore, very desirable.
Sterilization has a number of definitions including those promulgated by the regulating agencies of various jurisdictions and the various definitions adopted by medical research facilities. For purposes of this application, "sterilization" means conditions sufficient to obtain a log 10.sup.6 reduction of the spore forming bacteria, bacillus stearothennophilus. The description contained herein is of an apparatus and method which will achieve that standard. Obviously the apparatus and method can also achieve lower standards. For purposes of this application, "disinfection" means any treatment not meeting the requirements of sterilization.
The method of sterilization generally acknowledged to be most reliable is autoclaving, which entails the heating of items to at least about 212.degree. F. in a steam saturated atmosphere for periods of time ranging from about ten minutes to one day or more. Pressurized steam sterilization of instruments by autoclaving has been done by hospitals and medical offices for many years. However, large-scale sterilization of medical waste is possible only if steam is able to penetrate voluminous waste for a sufficient time at a sufficient pressure and temperature to effect essentially complete sterilization of all the wastes no matter what their location within the larger container. To this end, a pressure vessel is required in order to achieve sterilization as opposed to a vessel which is open to normal atmospheric conditions. After autoclaving or incineration which is sufficient to sterilize waste, residues can be deposited in landfills.
Another method of sterilization commonly used is dry heat. However, dry heat sterilization requires a lengthy period of heating. Other sterilization methods successfully used in limited situations include chemical vapor sterilization, bacteriocidal chemical treatment, and liquid disinfectant usage.
Recently, an interest has arisen in using microwave energy for sterilization. See, e.g., U.S. Pat. Nos. 5,098,665; 5,124,125; 5,213,758; 5,246,674; and 5,223,231. The devices of these patents transmit multimode microwaves. Thus, they are relatively inefficient in transmitting microwave energy to the load to be treated. In addition, these devices can only propagate or transmit the microwaves in a linear fashion.
Thus, there remains a continuing need for a quick, reliable and inexpensive way to locally sterilize infectious waste, and for a device which will permit the use of microwave energy in a pressure vessel. The present invention answers these needs and has the following advantages:
(a) it allows a more efficient use of microwaves to sterilize in a pressure vessel; PA1 (b) it increases the energy efficiency of microwave ovens; PA1 (c) it provides, in the sterilization embodiment, a device which can produce sufficient heat to sterilize throughout the entire volume of waste; PA1 (d) it provides, in the preferred sterilization embodiment, a compact apparatus that does not require drains; PA1 (e) it provides an automatic control of processing; and PA1 (f) it provides an apparatus that is highly efficient in transmitting energy to the containment vessel.